Inserting a rule into an empty style sheet shouldn't trigger style recalc unless necessary.
<http://webkit.org/b/119475>
<rdar://problem/14643481>

Reviewed by Antti Koivisto.

This is kind of a cheesy optimization, but it turns out that the use case is quite common.
The pattern goes like this:

    (1) Create <style> element.
    (2) Add it to the document's <head>.
    (3) .addRule() one rule through the CSSOM API.

Prior to this patch, (3) would always cause a full (deferred) style recalc.

Now that we exclude empty style sheets from the document's (effective) active set,
we can piggyback on the style invalidation analysis when transitioning from an empty
sheet to a single-rule sheet.

In other words, add a special code path for the first rule insertion into an empty,
in-document style sheet to minimize the amount of invalidation that happens.

* css/CSSStyleSheet.cpp:
(WebCore::CSSStyleSheet::didMutateRules):
(WebCore::CSSStyleSheet::insertRule):
* css/CSSStyleSheet.h:
(WebCore::CSSStyleSheet::RuleMutationScope::RuleMutationScope):
(WebCore::CSSStyleSheet::RuleMutationScope::~RuleMutationScope):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153699 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Document needn't expose its active element.
<http://webkit.org/b/119466>

Reviewed by Antonio Gomes.

The Document::m_activeElement pointer is only used inside updateHoverActiveState(),
so we can remove the activeElement()/setActiveElement() accessors.

* dom/Document.h:
* dom/Document.cpp:
(WebCore::Document::updateHoverActiveState):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153700 268f45cc-cd09-0410-ab3c-d52691b4dbfc